Electric vehicles (EVs) have advantages over current internal combustion engine vehicles (ICEVs) in that the EV helps solve energy problems and environmental problems and offers a torque response several hundred times as quick as that of the ICEV, enabling a generated torque to be accurately determined. These advantages relate to a motor and are inherent in the electric vehicle, which utilizes a motor as a driving force. Paying much attention to these advantages enables vehicle control that is impossible for the ICEV. Efforts have also been made to perform traction control based on these advantages of the motor (see Non-Patent Documents 1 and 2).
Measurement of a body speed is essential for measuring a slip ratio that is an important variable for implementing the traction control. The measurement of the body speed requires installation of a fifth wheel, or mounting of a sensor on a non-drive wheel for measurement of the rotation speed of the wheel, or integration of values obtained from an acceleration sensor.
However, disadvantageously, the installation of the fifth wheel is very difficult. Furthermore, if the acceleration sensor is used, noise or offset associated with integration may occur, making precise measurement difficult. The method of mounting the sensor on the non-drive wheel to measure the body speed based on the rotation speed of the wheel is also disadvantageous in that the body speed cannot be measured during braking, in that if the front wheels of the vehicle are non-drive wheels, the body speed cannot be accurately determined when a steering wheel is turned, and in that the method cannot be applied to four-wheel drive car having no non-drive wheel. Additionally, the sensor mounted on the non-drive wheel disadvantageously increases costs and complicates the system.
The present invention has been made in view of these problems. An object of the present invention is to provide a slip ratio estimating device that estimates the slip ratio without the need for the body speed and a slip ratio control device that uses the slip ratio estimating device to enable the slip ratio to quickly follow a target value.    Non-Patent Document 1: Kantaro YOSHIMOTO, Atsuo KAWAMURA, “Method of Estimating Wheel Slip Rate of Electric Vehicle”, Convention of the Industry Applications Department of the Institute of Electrical Engineers of Japan, 2000, Vol. 2, pp, 561-564    Non-Patent Document 2: Yoshimasa TSURUOKA, Yasushi TOYODA, Yoichi HORI, “Basic Study of Traction Control of Electric Vehicle”, Journal of the Institute of Electrical Engineers of Japan, 1998, Vol. 118-D, No. 1, pp. 44-50    Non-Patent Document 3: H. B. Pacejka, and E. Bakker, “The Magic Formula Tyre Model”, Proc. 1st International Colloquium on Tyre Models for Vehicle Dynamics Analysis, Held in Delft, The Netherlands, Oct. 21-22, 1991    Non-Patent Document 4: Takeo SAITO, Hiroshi FUJIMOTO, Suchiko NOGUCHI, “Method of Stabilizing and Controlling Steering of Electric Vehicle Using Slip Rate and Yaw Moment Observer”, 2003, Industrial Measurement Control Workshop of the Institute of Electrical Engineers of Japan, IIC-03-52, pp. 41-46    Non-Patent Document 5: Y. Hori, “Traction Control of Electric Vehicle-Basic Experimental Results using the Test EV “UOT Electric March II””, IEEE Trans. on Industry Applications, 1998, Vol. 34, No. 5, pp. 1131-1138    Non-Patent Document 6: Shinichiro SAKAI, Yoichi HORI, “Study of New Vehicle Motion Control of Electric Vehicle”, Dissertation of The University of Tokyo, 1999